Precision power-on reset circuit

ABSTRACT

A precision power-on reset circuit which is highly insensitive to temperature and process variations includes a self-biased proportional-to-absolute-temperature (PTAT) current generator 4, a base-emitter (V BE ) voltage detector 6, and a bipolar complementary metal oxide semiconductor (BiCMOS) inverter 8, which generates a power-on reset pulse for resetting an application circuit when a power supply voltage is turned on. The power-on reset circuit may further include a complementary metal oxide semiconductor (CMOS) buffer 10 coupled to the BiCMOS inverter 8 to isolate the application circuit from currents in the power-on reset circuit.

TECHNICAL FIELD

The present invention relates to a power-on reset circuit, and more particularly, to a bipolar complementary metal oxide semiconductor (BiCMOS) power-on reset circuit.

BACKGROUND ART

Power-on reset circuits are usually required on various types of semiconductor integrated circuit chips to initialize various blocks of application circuit elements such as state machines and essential memory elements, for example, the memory gates of a non-volatile memory device. The semiconductor integrated circuit industry has developed various types of conventional power-on reset circuits for generating a power-on reset pulse in response to the application of a direct current (DC) common voltage V_(cc) to the conventional power-on reset circuit. A typical conventional power-on reset circuit may be represented by a generic block diagram as shown in FIG. 1, with an input power supply voltage V_(cc) applied to the conventional power-on reset circuit 2, which in response generates a power-on reset (PORST) pulse with a voltage roughly proportional to the input voltage V_(cc) over a predetermined range of input power supply voltages.

When the integrated circuit is turned on, that is, when the supply voltage V_(cc) increases from 0 V to a DC supply voltage V_(cc) (DC), it is desirable that the voltage of the power-on reset pulse follow the supply voltage V_(cc) exactly, up to a predetermined upper threshold voltage level V_(cc) ', which is a voltage slightly lower than the DC supply voltage V_(cc) (DC). When the supply voltage V_(cc) reaches above the upper threshold voltage V_(cc) ', it is desirable that the power-on reset pulse terminate exactly when it reaches the voltage level V_(cc) '. It is also desirable that the power-on reset pulse start as early as possible at a minimum turn-on voltage V_(cc) °, after the supply voltage V_(cc) increases above the ground voltage 0 V. It is further desirable that the operating voltage range of the power-on reset pulse between the minimum turn-on voltage V_(cc) ° and the maximum pulse voltage V_(cc) ' be as insensitive to temperature and process variations as possible. However, the operating voltage ranges of power-on reset pulses generated by conventional power-on reset circuits are usually sensitive to variations due to the processing technology of semiconductor devices and to variations in operating temperatures.

In low voltage applications, the noise of the supply voltage V_(cc) may be appreciable compared to the DC supply voltage V_(cc) (DC). In this situation, the maximum pulse voltage V_(cc) ', which is the same as the upper threshold V_(cc) ' of the supply voltage V_(cc), may be set at a voltage level slightly lower than the DC voltage V_(cc) (DC) to take the noise voltage into the account. However, if the voltage V_(cc) ' at the end of the power-on reset pulse is too low, the application circuit which is to be reset by the power-on reset circuit may not be initialized properly. On the other hand, if the voltage V_(cc) ' is too close to the DC supply voltage V_(cc) (DC), the voltage V_(cc) ' may shift to a voltage level higher than the DC supply voltage V_(cc) (DC) due to the sensitivities of V_(cc) ' to temperature and process variations in a conventional power-on reset circuit. In either case, an application circuit to which the conventional power-on reset circuit is connected may not be able to function correctly because it is not properly initialized by the temperature and process sensitive power-on reset pulse generated by the conventional power-on reset circuit.

Therefore, there is a need for a power-on reset circuit that is able to generate a power-on reset pulse with an upper threshold voltage V_(cc) ' that is highly insensitive to temperature and process variations, particularly in situations in which the power-on reset circuit is implemented for generating power-on reset pulses in low voltage applications.

DISCLOSURE OF THE INVENTION

The present invention satisfies this need. In accordance with the present invention, a power-on reset circuit comprises:

(a) a self-biased current generator capable of receiving a supply voltage and generating a current which is proportional to an absolute temperature in the self-biased current generator in response to receiving the supply voltage;

(b) a base-emitter voltage detector coupled to the self-biased current generator;

(c) a bipolar complementary metal oxide semiconductor (BiCMOS) inverter coupled to the base-emitter voltage detector and capable of generating a power-on reset pulse having a voltage proportional to the supply voltage up to a predetermined threshold voltage; and

(d) a power-on reset output coupled to the BiCMOS inverter and capable of outputting the power-on reset pulse from the BiCMOS inverter.

In a further embodiment, the self-biased current generator comprises a first p-channel metal oxide semiconductor (PMOS) transistor, a second PMOS transistor, a first npn bipolar transistor, a second npn bipolar transistor, and a first resistor coupled to the second npn bipolar transistor which generates the current which is proportional to the absolute temperature.

In an additional embodiment, the base-emitter voltage detector comprises a third PMOS transistor, a third npn bipolar transistor, a fourth npn bipolar transistor, and a second resistor connected between the third and fourth npn bipolar transistors for generating a mirror image of the current which is proportional to the absolute temperature in the self-biased current generator. Furthermore, a capacitor may be coupled to the third npn bipolar transistor for prolonging the power-on reset pulse if the supply voltage increases rapidly during the turning on of the circuit.

In an additional embodiment, the BiCMOS inverter comprises a fourth PMOS transistor and a fifth npn bipolar transistor coupled to the third PMOS transistor and the fourth npn bipolar transistor, respectively, in the base-emitter voltage detector. In yet a further embodiment, the CMOS buffer comprises at least one pair of PMOS and n-channel metal oxide semiconductor (NMOS) transistors for buffering the power-on reset output from the current in the BiCMOS inverter. In order for the power-on reset circuit to generate a power-on reset pulse with a positive non-inverted voltage waveform, the CMOS buffer comprises two pairs of PMOS and NMOS transistors in a cascade following the BICMOS inverter.

Advantageously, the power-on reset circuit according to the present invention is capable of generating a power-on reset pulse with a tightly controlled operating voltage range defined by a predetermined upper threshold voltage which is highly insensitive to variations in the operating temperature and variations due to the processing technology. Another advantage of the present invention is that a power-on reset pulse with a sufficiently long duration is generated even if the power supply voltage increases very rapidly during the turning on of power supply.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with respect to particular embodiments thereof, and references will be made to the drawings in which:

FIG. 1, described above, is a generic block diagram of a power-on reset circuit which outputs a power-on reset (PORST) pulse in response to an input power supply voltage V_(cc) ;

FIG. 2 is a block diagram of a power-on reset circuit in accordance with the present invention;

FIG. 3 is a detailed circuit diagram of an embodiment of the power-on reset circuit in accordance with the present invention; and

FIG. 4 shows a typical transfer characteristic of the power-on reset (PORST) pulse voltage V_(p) versus the power supply voltage V_(cc) for the power-on reset circuit in accordance with the present invention.

MODES FOR CARRYING OUT THE INVENTION

The present invention provides a precision power-on reset circuit which is capable of generating a power-on reset pulse in response to the turning on of a power supply voltage over a predetermined operating voltage range that is highly insensitive to temperature and process variations. Moreover, the power-on reset circuit according to the present invention is capable of generating a power-on reset pulse with a sufficiently long duration even if the power-on ramp is very steep, that is, if the supply voltage increases very rapidly from the ground voltage to the direct current (DC) common supply voltage during the turning on of the power-on reset circuit.

FIG. 2 is a block diagram showing the generic circuit blocks of the power-on reset circuit in accordance with the present invention. The power-on reset circuit generally comprises a self-biased proportional-to-absolute-temperature (PTAT) current generator 4 which is coupled to receive a supply voltage V_(cc), a base-emitter voltage (V_(BE)) detector 6 coupled to the self-biased PTAT current generator 4, a bipolar complementary metal oxide semiconductor (BiCMOS) inverter 8 coupled to the base-emitter voltage detector 6, and a complementary metal oxide semiconductor (CMOS) buffer 10, which is coupled to the BiCMOS inverter 8 for generating the power-on reset pulse (PORST). The self-biased PTAT current generator 4 is capable of receiving the supply voltage V_(cc), which increases from 0 V to the common DC voltage V_(cc) (DC) when power supply for the power-on reset circuit is turned on. In response to receiving the supply voltage V_(cc), the self-biased current generator 4 generates a current I_(PTAT), which is a current proportional to the absolute temperature in the self-biased current generator 4.

The base-emitter voltage detector 6, which is coupled to the self-biased PTAT current generator 4, generates a current which is a mirror image of the current I_(PTAT) in the self-biased current generator 4. The BiCMOS inverter 8, which is coupled to the base-emitter voltage detector 6, is capable of generating a power-on reset (PORST) pulse having a voltage that is proportional to the supply voltage V_(cc) up to a predetermined upper-threshold voltage V_(cc) '. In order for an application circuit to be reset by the power-on reset pulse generated by the power-on reset circuit according to the present invention, the CMOS buffer 10 is coupled to the BiCMOS inverter 8 to isolate the current I_(PTAT) from the application circuit (not shown) which receives the power-on reset (PORST) pulse.

FIG. 3 is a detailed circuit diagram showing an embodiment of the power-on reset circuit of FIG. 2 according to the present invention. In this embodiment, the self-biased PTAT current generator 4 comprises a first p-channel metal oxide semiconductor (PMOS) transistor 12, a second PMOS transistor 14, a first npn bipolar transistor 16 and a second npn bipolar transistor 18. The source 12a of the first PMOS transistor 12 and the source 14a of the second PMOS transistor 14 are connected to receive the power supply voltage V_(cc). The gate 14c of the second PMOS transistor 14 is connected to the gate 12c of the first PMOS transistor 12, and is further connected to the drain 14b of the second PMOS transistor 14.

The collector 16b of the first npn bipolar transistor 16 is connected to the drain 12b of the first PMOS transistor 12, and is further connected to the base 16c of the first npn bipolar transistor 16. The emitter 16a of the first npn bipolar transistor 16 is connected to ground.

The collector 18b of the second npn bipolar transistor 18 is connected to the drain 14b of the second PMOS transistor 14, and the base 18c of the second npn bipolar transistor 18 is connected to the base 16c of the first npn bipolar transistor 16.

The self-biased PTAT current generator 4 further comprises a first resistor 20 with a resistance value R₁ coupled to the emitter 18a of the second npn bipolar transistor 18. The first resistor 20 has one end connected to the emitter 18a of the second npn bipolar transistor 18 and a second end which is connected to ground. In response to receiving the supply voltage V_(cc) by the sources 12a and 14a of the first and second PMOS transistors 12 and 14, respectively, the self-biased PTAT current generator 4 produces a current I_(PTAT) which is proportional to the absolute temperature of the self-biased current generator 4 at the emitter 18a of the second npn bipolar transistor 18. The current I_(PTAT) is a positive current which flows from the emitter 18a of the second npn bipolar transistor 18 to ground through the first resistor 20.

Furthermore, the base-emitter voltage (V_(BE)) detector 6 in the power-on reset circuit according to the present invention comprises a third PMOS transistor 22, a third npn bipolar transistor 24 and a fourth npn bipolar transistor 26. The source 22a of the third PMOS transistor 22 is connected to receive the supply voltage V_(cc), and the gate 22c of the third PMOS transistor 22 is connected to the gates 12c and 14c of the first and second PMOS transistors 12 and 14, respectively. The collector 24b of the third npn bipolar transistor 24 is connected to the drain 22b of the third PMOS transistor 22, and is further connected to the base 24c of the third npn bipolar transistor 24. The base 26c of the fourth npn bipolar transistor 26 is connected to the bases 16c and 18c of the first and second npn bipolar transistors 16 and 18, respectively. The emitter 26a of the fourth npn bipolar transistor 26 is connected to ground.

The base-emitter voltage detector 6 further comprises a second resistor 28 with a resistance value R₂, which is connected between the emitter 24a of the third npn bipolar transistor 24 and the collector 26b of the fourth npn bipolar transistor 26. The second resistor 28 and the first resistor 20 may be made of the same type of materials using the same processing technology such that the ratio of the resistances of the first and second resistors 20 and 28 is insensitive to temperature and process variations. A current with a value of I_(PTAT), which is a mirror image of the current that flows through the first resistor 20 in the self-biased PTAT current generator 4, flows from the emitter 24a of the third npn bipolar transistor 24 to the collector 26b of the fourth npn bipolar transistor 26 through the second resistor 28.

Furthermore, in order for the power-on reset circuit to generate a power-on reset pulse with a sufficiently long pulse duration in a situation in which the power-on ramp of the supply voltage V_(cc) is very steep during the turning on of the power-on reset circuit, a capacitor 30 is coupled to the emitter 24a of the third npn bipolar transistor 24. The capacitor 30 has one end which is connected to the emitter 24a of the third npn bipolar transistor 24 and a second end which is connected to ground.

In accordance with the present invention, the BiCMOS inverter 8 comprises a fourth PMOS transistor 32 and a fifth npn bipolar transistor 34. The source 32a of the PMOS transistor 32 is connected to receive the supply voltage V_(cc), and the gate 32c of the fourth PMOS transistor 32 is connected to the gates 12c, 14c and 22c of the first, second and third PMOS transistors 12, 14 and 22, respectively. The collector 34b of the fifth npn bipolar transistor 34 is connected to the drain 32b of the fourth PMOS transistor 32. The base 34c of the fifth npn bipolar transistor 34 is connected to the collector 26b of the fourth npn bipolar transistor 26 in the base-emitter voltage detector 6. The emitter 34a of the fifth npn bipolar transistor 34 is connected to ground.

In a further embodiment, the power-on reset circuit according to the present invention further comprises a CMOS buffer 10 which is coupled to the BiCMOS inverter 8 to output the power-on reset pulse and to isolate the application circuit (not shown) to which the power-on reset circuit according to the present invention is connected from the currents in the power-on reset circuit. The CMOS buffer 10 comprises at least one pair of PMOS and NMOS transistors including a fifth PMOS transistor 36 and an n-channel metal oxide semiconductor (NMOS) transistor 38. The source 36a of the fifth PMOS transistor 36 is connected to receive the supply voltage V_(cc), whereas the source 38a of the first NMOS transistor 38 is connected to ground. The drain 36b of the fifth PMOS transistor 36 is connected to the drain 38b of the first NMOS transistor 38, and the gate 36c of the fifth PMOS transistor 36 is connected to the gate 38c of the first NMOS transistor 38. Furthermore, the gates 36c and 38c of the fifth PMOS transistor 36 and the first NMOS transistor 38, which are connected together, are connected to the collector 34b of the fifth npn bipolar transistor 34 and the drain 32b of the fourth PMOS transistor 32, which are connected together in the BiCMOS inverter 8.

In order for the voltage V_(p) of the output power-on reset (PORST) pulse to have a non-inverted positive voltage waveform, the CMOS buffer 10 further includes an additional pair of PMOS and NMOS transistors comprising a sixth PMOS transistor 40 and a second NMOS transistor 42. The source 40a of the sixth PMOS transistor 40 is connected to receive the supply voltage V_(cc) and the source 42a of the second NMOS transistor 42 is connected to ground. The drain 40b of the sixth PMOS transistor 40 is connected to the drain 42b of the second NMOS transistor 42, and the gate 40c of the sixth PMOS transistor 40 is connected to the gate 42c of the second NMOS transistor 42. The gates 40c and 42c of the sixth PMOS transistor 40 and the second NMOS transistor 42, which are connected together, are connected to the drains 36b and 38b of the fifth PMOS transistor 36 and the first NMOS transistor 38, which are connected together. The drains 40b and 42b of the sixth PMOS transistor 40 and the second NMOS transistor 42, which are connected together, are connected to a power-on reset (PORST) output terminal 44 for outputting the PORST pulse signal with a positive pulse voltage V_(p). An application circuit such as a state machine or a memory circuit may be connected to the PORST output 44 to receive the power-on reset pulse, which resets the application circuit during the turning on of the power supply.

In yet a further embodiment, additional pairs of PMOS and NMOS transistors may be cascaded together in the CMOS buffer 10 to provide further isolation for the PORST output 44 from the currents within the power-on reset circuit. In order for the output PORST signal to have a positive non-inverted voltage waveform, it is desirable that an even number of pairs of PMOS and NMOS transistors be included in the CMOS buffer 10.

In a further embodiment, the first, second and third PMOS transistors 12, 14 and 22 have the same shape factor, defined as the ratio of channel width to channel length (W/L), which is a physical characteristic of a MOS transistor known to a person skilled in the art, whereas the fourth PMOS transistor 32 has a shape factor different from that of the first, second and third PMOS transistors 12, 14 and 22. For example, the shape factor W/L of the fourth PMOS transistor 32 may be roughly one-quarter of that of each of the first, second and third PMOS transistors 12, 14 and 22. The shape factors of the PMOS transistors 36 and 40 and of the NMOS transistors 38 and 42 in the CMOS buffer 10 may be the same as or different from the shape factor (W/L) of the first, second and third PMOS transistors 12, 14 and 22, and are thus not critical to the present invention.

In a further embodiment, the first, third, fourth and fifth npn bipolar transistors 16, 24, 26 and 34 have an equal emitter area whereas the second npn bipolar transistor 18 has an emitter area different from that of each of the first, third, fourth and fifth npn bipolar transistors 16, 24, 26 and 34. The first, third, fourth and fifth transistors 16, 24, 26 and 34 each have a unit-size emitter area while the emitter area of the second npn bipolar transistor 18 is A times the unit-size emitter area of each of the first, third, fourth and fifth npn bipolar transistors 16, 24, 26 and 34. As discussed below, the size factor A for the emitter area of the second npn bipolar transistor 18 is an important parameter in the design of the power-on reset circuit according to the present invention.

FIG. 4 shows a typical transfer characteristic of the voltage V_(p) of the power-on reset (PORST) pulse versus the power supply voltage V_(cc), which is the common input power voltage for the power-on reset circuit according to the present invention.

As shown in FIG. 4, the PORST pulse is generated when the supply voltage V_(cc) is in the range of V_(cc) ° to V_(cc) '. The PORST pulse begins when the supply voltage V_(cc) reaches the lower threshold voltage V_(cc) °, and increases linearly with the supply voltage V_(cc) until the supply voltage V_(cc) reaches the upper threshold voltage V_(cc) ', the point at which the PORST pulse ends and the output voltage V_(p) of the power-on reset circuit drops to 0 V. The power supply voltage V_(cc) increases further from the upper threshold voltage V_(cc) ' to the DC power supply voltage V_(cc) (DC), which is the operating DC voltage for the application circuit connected to receive the PORST pulse generated by the power-on reset circuit.

The lowest threshold voltage V_(cc) ° should be as low as possible such that the relevant circuit elements in the application circuit can be initialized by the PORST pulse as early as possible during the turning on of the power supply voltage V_(cc). In the power-on reset circuit according to the present invention, the lower threshold voltage V_(cc) ° is typically in the range of about 1.2 V to 1.4 V for a DC supply voltage V_(cc) (DC) on the order of about 3 V, and is acceptably close to the ground voltage in many low voltage applications. Although the lower threshold voltage V_(cc) ° is ideally set at 0 V, in practice it is limited by the turn-on voltage of a PMOS transistor which is typically in the range of about 0.8 V to 1.0 V.

The upper threshold voltage V_(cc) ' should satisfy the following two-sided relationship:

    V.sub.cc '(min)≦V.sub.cc '≦V.sub.cc (max)    (1)

wherein V_(cc) ' (min) is the minimum value of the supply voltage V_(cc) required for the relevant circuit elements to function correctly for proper initialization. The maximum upper threshold voltage V_(cc) ' (max) can be determined by the following relationship:

    V.sub.cc (max)=V.sub.ccmin (DC)-Δ.sub.noise          (2)

wherein V_(ccmin) (DC) is the minimum value of the DC supply voltage for maintaining normal operation of the application circuit and Δ_(noise) is a noise margin parameter, which can be estimated or measured by a person skilled in the art.

In order to achieve a tight control on the spread in the upper threshold voltage V_(cc) ' over temperature and process variations in low voltage applications in which the difference between the voltage V_(cc) ' (max) and the voltage V_(cc) ' (min) is typically small, the power-on reset circuit according to the present invention is able to generate a PORST pulse with an upper threshold voltage V_(cc) ' that is highly insensitive to temperature and process variations by selecting the proper values of the size factor A for the emitter area of the second npn bipolar transistor 18 and the resistances R₁ and R₂ of the first and second resistors 20 and 28, respectively, as discussed below. In the self-biased PTAT current generator 4, for a sufficiently large value of V_(cc), the current I_(PTAT), which is the current flowing from the emitter 18a of the second npn bipolar transistor 18 through the first resistor 20, is approximately given by the relationship: ##EQU1## wherein V_(BE) (16) and V_(BE) (18) are the base-emitter voltages of the first and second npn bipolar transistors 16 and 18, respectively. The size factor A for the second npn bipolar transistor 18 is the ratio of the emitter area of the second npn bipolar transistor 18 to the emitter area of the first npn bipolar transistor 16. In Equation (3), q is the electronic charge, k is Boltzmann's constant, T is the absolute temperature of the self-biased PTAT current generator 4, and R₁ is the resistance value of the first resistor 20. It is apparent from Equation (3) that the size factor A for the emitter area of the second npn bipolar transistor 18 should be other than one in order to have a non-zero current I_(PTAT) to flow through the first resistor 20. It is further apparent from Equation (3) that the emitter area of the second npn bipolar transistor 18 should be larger than that of the first npn bipolar transistor 16 such that the current I_(PTAT) flowing from the emitter 18a of the second npn bipolar transistor 18 to the first resistor 20 is a positive current.

Assuming that the supply voltage V_(cc) is ramped up from the ground voltage of 0 V during the turning on of the power supply, the current I_(PTAT) begins to flow through the first resistor 20 as soon as the supply voltage V_(cc) reaches the lower threshold voltage V_(cc) °. As the supply voltage V_(cc) increases beyond the lower threshold voltage V_(cc) °, the voltage V_(p) of the output PORST pulse follows the input supply voltage V_(cc) linearly until the supply voltage V_(cc) reaches the upper threshold voltage V_(cc) '. As shown in FIG. 4, the PORST pulse voltage V_(p) is equal to the supply voltage V_(cc) when V_(cc) is in the range of V_(cc) ° to V_(cc) ', as illustrated by the 45° line segment from V_(cc) ° to V_(cc) '.

Referring back to FIG. 3, the current I_(PTAT), which is proportional to the absolute temperature T in the self-biased current generator 4 according to Equation (3) above, is reproduced as a mirror image current I_(PTAT) through the second resistor 28 in the base-emitter voltage detector 6. The current I_(PTAT) thus flows through the third PMOS transistor 22, the third npn bipolar transistor 24, the second resistor 28, and the fourth npn bipolar transistor 26. In a quasi-DC circuit analysis, no current flows through the capacitor 30, which is connected to the emitter 24a of the third npn bipolar transistor 24. The third PMOS transistor 22 has the same shape factor W/L as that of the first and second PMOS transistors 12 and 14, while the third and fourth npn bipolar transistors 24 and 26 each have an emitter area equal to that of the first npn bipolar transistor 16. The voltage across the base-emitter voltage detector 6, which is a voltage at the source 22a of the third PMOS transistor 22 relative to the ground voltage at the emitter 26a of the fourth npn bipolar transistor 26, is summed according to the following relationship:

    V.sub.cc =Δ+V.sub.BE (24)+I.sub.PTAT ·R.sub.2 +V.sub.1 (4)

wherein Δ is the drain-to-source voltage across the third PMOS transistor 22, V_(BE) (24) is the base-emitter voltage of the third npn bipolar transistor 24, and V₁ is the voltage at the collector 26b of the fourth npn bipolar transistor 26 with respect to ground. During the initial ramp-up of the supply voltage V_(cc), V₁ is small and is not sufficient to turn on the fifth npn bipolar transistor 34 in the BiCMOS inverter 8 for small values of V_(cc). The voltage at the collector 34b of the fifth npn bipolar transistor 34 and the drain 32b of the PMOS transistor 32, which are connected together, is thus driven to the supply voltage V_(cc) by the fourth PMOS transistor 32 through current mirroring action. The output voltage of the PORST pulse will thus follow the supply voltage V_(cc) for small values of V_(cc) greater than V_(cc) °.

When the supply voltage V_(cc) reaches a sufficiently large value such that the voltage V₁ reaches the base-emitter turn-on voltage V_(BE) (34) at the fifth npn bipolar transistor 34, the voltage V₁ causes the fifth npn bipolar transistor 34 to turn on and to pull the voltage V₂ at the collector 34b of the fifth npn bipolar transistor 34 to the ground voltage. Solving for the value of V_(cc) at which V₁ reaches V_(BE) (34) by using Equation (4), the following relationship is obtained:

    V.sub.cc '=Δ'+V.sub.BE (24)+I.sub.PTAT ·R.sub.2 +V.sub.BE (34)                                                      (5)

Since the third and fifth npn bipolar transistors 24 and 34 have an identical emitter area, the base-emitter turn-on voltages V_(BE) (24) and V_(BE) (34) are the same, and can thus be denoted as V_(BE). Furthermore, by substituting the term I_(PTAT) in Equation (5) by Equation (3), the following relationship is obtained: ##EQU2##

The term Δ' in Equation (6) above is the drain-to-source voltage V_(DS) for the third PMOS transistor 22, and is relatively insensitive to variations in the supply voltage V_(cc). Although the voltage V_(DS) for a PMOS transistor is dependent on the processing technology to some degree, in practice the voltage Δ' is a small voltage on the order of about 0.1 V. Compared to the other two terms in Equation (6), the effect of Δ' on the upper threshold voltage V_(cc) ' is relatively small.

Assuming that Δ' is insensitive to V_(cc), it is apparent from Equation (6) that the upper threshold voltage V_(cc) ' is only a function of V_(BE), T and R₂ /R₁. Since it is known to a person skilled in the art that the base-emitter turn-on voltage V_(BE) of a bipolar junction transistor, which is typically in the range of about 0.6 V to 0.7 V_(cc) is relatively insensitive to process variations, that the ratio of two resistors of the same type produced by the same processing technology on an integrated circuit is highly process and temperature insensitive, and that the size factor A, which is the ratio of the emitter area of the second npn bipolar transistor 18 to the emitter area of the first npn bipolar transistor 16, is also highly process insensitive, it follows that the upper threshold voltage V_(cc) ' is relatively process insensitive.

Further assuming that Δ' is insensitive to process and temperature variations and is therefore constant with respect to temperature, by taking the partial derivative of Equation (6) with respect to the temperature, the following relationship is obtained: ##EQU3##

Since it is known to a person skilled in the art that the base-emitter voltage V_(BE) has a negative temperature coefficient, which is typically on the order of about -2 mV/°C. for a typical npn bipolar junction transistor, it follows that the partial derivative of V_(BE) with respect to the temperature is less than zero. Hence, in Equation (7), the upper threshold voltage V_(cc) ' can be temperature compensated because ##EQU4## such that the derivative of V_(cc) ' with respect to the temperature is the sum of a positive and a negative number. By adjusting the ratio R₂ /R₁ and the size factor A for the emitter area of the second npn bipolar transistor 18, the partial derivative V_(cc) ' with respect to the temperature can be made to approximately equal zero at a suitable value of V_(cc) '.

Referring back To Equation (6), the term Δ', which is relatively process and temperature insensitive, is negligibly small. The term 2 V_(BE) is a significant term which typically has a value in the range of about 1.2 V to 1.4 V but in practice is relatively insensitive to the processing technology. As an illustrative example, by using a value of 0.65 V for the base-emitter voltage V_(BE), a value of 13 for the ratio R₂ /R₁, and a value of 12 for the emitter size factor A, and assuming that the power-on reset circuit operates at a temperature of 322°K., and further assuming that Δ' equals 0.1 V, the resulting upper threshold voltage V_(cc) ' is approximately 2.3 V, which in practice should be acceptable for an application circuit operating with a minimum DC supply voltage V_(ccmin) (DC) in the range of about 2.7 V to 3.0 V.

Based upon circuit simulations, it is found that the upper threshold voltage V_(cc) ' varies by approximately ±5% over a temperature range of 0° C. to 80° C. (273° K. to 353° K.) and over a wide range of process variations. Therefore, in the operating example described above, the typical upper threshold voltage V_(cc) ' is set at 2.3 V and varies from 2.19 V to 2.42 V over wide process and temperature variations. In practice, this variation in V_(cc) ' is quite acceptable for a DC supply voltage V_(cc) (DC) with a value of 3.0 V ±10%. It is also found that the lower threshold voltage V_(cc) ° is in the range of about 1.2 V to 1.4 V, which is quite acceptable in many practical applications in which the DC supply voltage is on the order of about 3 V.

Furthermore, the capacitor 30 may be provided in the base-emitter voltage detector 6 as shown in FIG. 3 to assure a PORST pulse of at least a sufficiently long duration to reset the application circuit if the on-ramp of the power supply voltage V_(cc) is very steep, that is, if the supply voltage V_(cc) is turned on very rapidly from 0 V to V_(cc) (DC). The duration of the PORST pulse can be approximated by the time constant τ which is equal to R₂ C. Selection of the time constant τ depends on the duration of the PORST pulse required for resetting the application circuit to which the power-on reset circuit according to the present invention is connected during the turning on of the power supply voltage V_(cc). For example, the time constant τ may be on the order of a few microseconds for resetting a conventional memory circuit, and the capacitance value C of the capacitor 30 may be selected based on the time constant τ and the resistance value R₂ of the second resistor 28 in the base-emitter voltage detector 6.

INDUSTRIAL APPLICABILITY

The precision power-on reset circuit according to the present invention is applicable for the power-on reset of a variety of integrated circuits, such as state machines and memory devices including non-volatile memory devices. The power-on reset circuit according to the present invention generates a power-on reset pulse that is highly insensitive to variations in the operating temperature and variations due to the processing technology. The power-on reset circuit according to the present invention may be implemented with the application circuit on the same semiconductor chip. Alternatively, the power-on reset circuit according to the present invention may be implemented on a semiconductor chip that is separate from the application circuit which receives the power-on reset pulse.

The present invention has been described with respect to particular embodiments thereof, and numerous modifications can be made which are within the scope of the invention as set forth in the claims. 

What is claimed is:
 1. A power-on reset circuit, comprising:(a) a self-biased current generator capable of receiving a supply voltage and generating a current which is proportional to an absolute temperature in the self-biased current generator in response to receiving the supply voltage; (b) a base-emitter voltage detector coupled to the self-biased current generator; (c) a bipolar complementary metal oxide semiconductor (BiCMOS) inverter coupled to the base-emitter voltage detector and capable of generating a power-on reset pulse having a voltage proportional to the supply voltage up to a predetermined threshold voltage; and (d) a power-on reset output coupled to the BiCMOS inverter and capable of outputting the power-on reset pulse from the BiCMOS inverter.
 2. The circuit of claim 1, wherein the self-biased current generator comprises:(i) a first p-channel metal oxide semiconductor (PMOS) transistor comprising a source, a drain and a gate, the source of the first PMOS transistor capable of receiving the supply voltage; (ii) a second PMOS transistor comprising a source, a drain and a gate, the source of the second PMOS transistor capable of receiving the supply voltage, and the gate of the second PMOS transistor connected to the drain of the second PMOS transistor and to the gate of the first PMOS transistor; (iii) a first npn bipolar transistor comprising an emitter, a collector and a base, the emitter of the first npn bipolar transistor capable of being grounded, and the collector of the first npn bipolar transistor connected to the base of the first npn bipolar transistor and to the drain of the first PMOS transistor; and (iv) a second npn bipolar transistor comprising an emitter, a collector and a base, the collector of the second npn bipolar transistor connected to the drain of the second PMOS transistor, and the base of the second npn bipolar transistor connected to the base of the first npn bipolar transistor.
 3. The circuit of claim 2, wherein the emitter of the second npn bipolar transistor has an emitter area larger than that of the emitter of the first npn bipolar transistor.
 4. The circuit of claim 2, wherein the first and second PMOS transistors have an equal shape factor.
 5. The circuit of claim 2, wherein the self-biased current generator further comprises a first resistor coupled to the emitter of the second npn bipolar transistor, the second npn bipolar transistor capable of supplying the current which is proportional to the absolute temperature to the first resistor in response to receiving the supply voltage by the sources of the first and second PMOS transistors.
 6. The circuit of claim 5, wherein the first resistor comprises a first terminal connected to the emitter of the second npn bipolar transistor and a second terminal capable of being grounded.
 7. The circuit of claim 1, wherein the base-emitter voltage detector comprises:(i) a third PMOS transistor comprising a source, a drain and a gate, the source of the third PMOS transistor capable of receiving the supply voltage; (ii) a third npn bipolar transistor comprising an emitter, a collector and a base, the collector of the third npn bipolar transistor connected to the drain of the third PMOS transistor and to the base of the third npn bipolar transistor; and (iii) a fourth npn bipolar transistor coupled to the third npn bipolar transistor, the fourth npn bipolar transistor comprising an emitter, a collector and a base, the emitter of the fourth npn bipolar transistor capable of being grounded.
 8. The circuit of claim 7, wherein the emitters of the third and fourth npn bipolar transistors have an equal emitter area.
 9. The circuit of claim 7, wherein the base-emitter voltage detector further comprises a second resistor connected between the emitter of the third npn bipolar transistor and the collector of the fourth npn bipolar transistor.
 10. The circuit of claim 9, wherein the base-emitter voltage detector further comprises a capacitor coupled to the emitter of the third npn bipolar transistor.
 11. The circuit of claim 10, wherein the capacitor comprises a first terminal connected to the emitter of the third npn bipolar transistor and a second terminal capable of being grounded.
 12. The circuit of claim 1, wherein the BiCMOS inverter comprises:(i) a fourth PMOS transistor comprising a source, a drain and a gate, the source of the fourth PMOS transistor capable of receiving the supply voltage; and (ii) a fifth npn bipolar transistor comprising an emitter, a collector and a base, the collector of the fifth npn bipolar transistor connected to the drain of the fourth PMOS transistor, and the emitter of the fifth npn bipolar transistor capable of being grounded.
 13. The circuit of claim 1, further comprising a complementary metal oxide semiconductor (CMOS) buffer coupled between the BiCMOS inverter and the power-on reset output.
 14. The circuit of claim 13, wherein the CMOS buffer comprises:(i) a fifth PMOS transistor comprising a source, a drain and a gate, the source of the fifth PMOS transistor capable of receiving the supply voltage; and (ii) a first n-channel metal oxide semiconductor (NMOS) transistor comprising a source, a drain and a gate, the source of the first NMOS transistor capable of being grounded, the drain of the first NMOS transistor connected to the drain of the fifth PMOS transistor, and the gate of the first NMOS transistor connected to the gate of the fifth PMOS transistor.
 15. The circuit of claim 14, wherein the CMOS buffer further comprises:(iii) a sixth PMOS transistor comprising a source, a drain and a gate, the source of the sixth PMOS transistor capable of receiving the supply voltage; and (iv) a second NMOS transistor comprising a source, a drain and a gate, the source of the second NMOS transistor capable of being grounded, the drain of the second NMOS transistor connected to the drain of the sixth PMOS transistor, and the gate of the second NMOS transistor connected to the gate of the sixth PMOS transistor and to the drains of the first NMOS transistor and the fifth PMOS transistor.
 16. The circuit of claim 15, wherein the power-on reset output is connected to the drains of the second NMOS transistor and the sixth PMOS transistor.
 17. The circuit of claim 1, wherein the voltage of the power-on reset pulse is equal to the supply voltage up to the predetermined threshold voltage.
 18. A power-on reset circuit, comprising:(a) a self-biased current generator capable of receiving a supply voltage and generating a current which is proportional to an absolute temperature in the self-biased current generator in response to receiving the supply voltage, the self-biased current generator comprising:(i) a first p-channel metal oxide semiconductor (PMOS) transistor comprising a source, a drain and a gate, the source of the first PMOS transistor capable of receiving the supply voltage; (ii) a second PMOS transistor comprising a source, a drain and a gate, the source of the second PMOS transistor capable of receiving the supply voltage, and the gate of the second PMOS transistor connected to the drain of the second PMOS transistor and to the gate of the first PMOS transistor; (iii) a first npn bipolar transistor comprising an emitter, a collector and a base, the emitter of the first npn bipolar transistor capable of being grounded, and the collector of the first npn bipolar transistor connected to the base of the first npn bipolar transistor and to the drain of the first PMOS transistor; (iv) a second npn bipolar transistor comprising an emitter, a collector and a base, the collector of the second npn bipolar transistor connected to the drain of the second PMOS transistor, and the base of the second npn bipolar transistor connected to the base of the first npn bipolar transistor; and (v) a first resistor coupled to the emitter of the second npn bipolar transistor, the second npn bipolar transistor capable of supplying the current which is proportional to the absolute temperature to the first resistor in response to receiving the supply voltage by the sources of the first and second PMOS transistors; (b) a base-emitter voltage detector coupled to the self-biased current generator; (c) a bipolar complementary metal oxide semiconductor (BiCMOS) inverter coupled to the base-emitter voltage detector; (d) a complementary metal oxide semiconductor (CMOS) buffer coupled to the BiCMOS inverter and capable of generating a power-on reset pulse having a voltage proportional to the supply voltage up to a predetermined threshold voltage; and (e) a power-on reset output coupled to the CMOS buffer and capable of outputting the power-on reset pulse from the CMOS buffer.
 19. The circuit of claim 18, wherein the emitter of the second npn bipolar transistor has an emitter area larger than that of the emitter of the first npn bipolar transistor.
 20. The circuit of claim 18, wherein the first and second PMOS transistors have an equal shape factor.
 21. The circuit of claim 18, wherein the first resistor comprises a first terminal connected to the emitter of the second npn bipolar transistor and a second terminal capable of being grounded.
 22. The circuit of claim 18, wherein the base-emitter voltage detector comprises:(i) a third PMOS transistor comprising a source, a drain and a gate, the source of the third PMOS transistor capable of receiving the supply voltage, and the gate of the third PMOS transistor connected to the gate of the second PMOS transistor; (ii) a third npn bipolar transistor comprising an emitter, a collector and a base, the collector of the third npn bipolar transistor connected to the drain of the third PMOS transistor and to the base of the third npn bipolar transistor; and (iii) a fourth npn bipolar transistor coupled to the third npn bipolar transistor, the fourth npn bipolar transistor comprising an emitter, a collector and a base, the emitter of the fourth npn bipolar transistor capable of being grounded, and the base of the fourth npn bipolar transistor connected to the base of the second npn bipolar transistor.
 23. The circuit of claim 22, wherein the emitters of the first, third and fourth npn bipolar transistors have an equal emitter area.
 24. The circuit of claim 22, wherein the first, second and third PMOS transistors have an equal shape factor.
 25. The circuit of claim 22, wherein the base-emitter voltage detector further comprises a second resistor connected between the emitter of the third npn bipolar transistor and the collector of the fourth npn bipolar transistor.
 26. The circuit of claim 25, wherein the base-emitter voltage detector further comprises a capacitor coupled to the emitter of the third npn bipolar transistor.
 27. The circuit of claim 26, wherein the capacitor comprises a first terminal connected to the emitter of the third npn bipolar transistor and a second terminal capable of being grounded.
 28. The circuit of claim 18, wherein the BiCMOS inverter comprises:(i) a fourth PMOS transistor comprising a source, a drain and a gate, the source of the fourth PMOS transistor capable of receiving the supply voltage; and (ii) a fifth npn bipolar transistor comprising an emitter, a collector and a base, the collector of the fifth npn bipolar transistor connected to the drain of the fourth PMOS transistor, and the emitter of the fifth npn bipolar transistor capable of being grounded.
 29. The circuit of claim 28, wherein the emitters of the first and fifth npn bipolar transistors have an equal emitter area.
 30. The circuit of claim 28, wherein the fourth PMOS transistor has a shape factor of about 1/4 of that of the first PMOS transistor.
 31. The circuit of claim 28, wherein the CMOS buffer comprises:(i) a fifth PMOS transistor comprising a source, a drain and a gate, the source of the fifth PMOS transistor capable of receiving the supply voltage; and (ii) a first n-channel metal oxide semiconductor (NMOS) transistor comprising a source, a drain and a gate, the source of the first NMOS transistor capable of being grounded, the drain of the first NMOS transistor connected to the drain of the fifth PMOS transistor, and the gate of the first NMOS transistor connected to the gate of the fifth PMOS transistor and to the collector of the fifth npn bipolar transistor.
 32. The circuit of claim 31, wherein the CMOS buffer further comprises:(iii) a sixth PMOS transistor comprising a source, a drain and a gate, the source of the sixth PMOS transistor capable of receiving the supply voltage; and (iv) a second NMOS transistor comprising a source, a drain and a gate, the source of the second NMOS transistor capable of being grounded, the drain of the second NMOS transistor connected to the drain of the sixth PMOS transistor, and the gate of the second NMOS transistor connected to the gate of the sixth PMOS transistor and to the drains of the first NMOS transistor and the fifth PMOS transistor.
 33. The circuit of claim 32, wherein the power-on reset output is connected to the drains of the second NMOS transistor and the sixth PMOS transistor.
 34. The circuit of claim 18, wherein the voltage of the power-on reset pulse is equal to the supply voltage up to the predetermined threshold voltage.
 35. A power-on reset circuit, comprising:(a) a self-biased current generator capable of receiving a supply voltage and generating a current which is proportional to an absolute temperature in the self-biased current generator in response to receiving the supply voltage; (b) a base-emitter voltage detector, coupled to the self-biased current generator, comprising:(i) a first p-channel metal oxide semiconductor (PMOS) transistor comprising a source, a drain and a gate, the source of the first PMOS transistor capable of receiving the supply voltage; (ii) a first npn bipolar transistor comprising an emitter, a collector and a base, the collector of the first npn bipolar transistor connected to the drain of the first PMOS transistor and to the base of the first npn bipolar transistor; and (iii) a second npn bipolar transistor comprising an emitter, a collector and a base, the emitter of the second npn bipolar transistor capable of being grounded; and (iv) a first resistor connected between the emitter of the first npn bipolar transistor and the collector of the second npn bipolar transistor; (c) a bipolar complementary metal oxide semiconductor (BiCMOS) inverter coupled to the base-emitter voltage detector; (d) a complementary metal oxide semiconductor (CMOS) buffer coupled to the BiCMOS inverter and capable of generating a power-on reset pulse having a voltage proportional to the supply voltage up to a predetermined threshold voltage; and (e) a power-on reset output coupled to the CMOS buffer and capable of outputting the power-on reset pulse from the CMOS buffer.
 36. The circuit of claim 35, wherein the self-biased current generator comprises:(i) a second PMOS transistor comprising a source, a drain and a gate, the source of the second PMOS transistor capable of receiving the supply voltage; (ii) a third PMOS transistor comprising a source, a drain and a gate, the source of the third PMOS transistor capable of receiving the supply voltage, the gate of the third PMOS transistor connected to the drain of the third PMOS transistor and to the gates of the first and second PMOS transistors; (iii) a third npn bipolar transistor comprising an emitter, a collector and a base, the emitter of the third npn bipolar transistor capable of being grounded, and the collector of the third npn bipolar transistor connected to the base of the third npn bipolar transistor and to the drain of the second PMOS transistor; and (iv) a fourth npn bipolar transistor comprising an emitter, a collector and a base, the collector of the fourth npn bipolar transistor connected to the drain of the third PMOS transistor, and the base of the fourth npn bipolar transistor connected to the bases of the second and third npn bipolar transistors.
 37. The circuit of claim 36, wherein the emitters of the first, second and third npn bipolar transistors have an equal emitter area.
 38. The circuit of claim 37, wherein the emitter of the fourth npn bipolar transistor has an emitter area larger than that of the emitter of the first npn bipolar transistor.
 39. The circuit of claim 36, wherein the first, second and third PMOS transistors have an equal shape factor.
 40. The circuit of claim 36, wherein the self-biased current generator further comprises a second resistor coupled to the emitter of the fourth npn bipolar transistor, the fourth npn bipolar transistor capable of supplying the current which is proportional to the absolute temperature to the second resistor in response to receiving the supply voltage by the sources of the second and third PMOS transistors.
 41. The circuit of claim 40, wherein the second resistor comprises a first terminal connected to the emitter of the fourth npn bipolar transistor and a second terminal capable of being grounded.
 42. The circuit of claim 35, wherein the base-emitter voltage detector further comprises a capacitor coupled to the emitter of the first npn bipolar transistor.
 43. The circuit of claim 42, wherein the capacitor comprises a first terminal connected to the emitter of the first npn bipolar transistor and a second terminal capable of being grounded.
 44. The circuit of claim 35, wherein the BiCMOS inverter comprises:(i) a fourth PMOS transistor comprising a source, a drain and a gate, the source of the fourth PMOS transistor capable of receiving the supply voltage, and the gate of the fourth PMOS transistor connected to the gate of the first PMOS transistor; and (ii) a fifth npn bipolar transistor comprising an emitter, a collector and a base, the collector of the fifth npn bipolar transistor connected to the drain of the fourth PMOS transistor, the emitter of the fifth npn bipolar transistor capable of being grounded, and the base of the fifth npn bipolar transistor connected to the collector of the second npn bipolar transistor.
 45. The circuit of claim 44, wherein the emitters of the first and fifth npn bipolar transistors have an equal emitter area.
 46. The circuit of claim 44, wherein the fourth PMOS transistor has a shape factor of about 1/4 of that of the first PMOS transistor.
 47. The circuit of claim 44, wherein the CMOS buffer comprises:(i) a fifth PMOS transistor comprising a source, a drain and a gate, the source of the fifth PMOS transistor capable of receiving the supply voltage; and (ii) a first n-channel metal oxide semiconductor (NMOS) transistor comprising a source, a drain and a gate, the source of the first NMOS transistor capable of being grounded, the drain of the first NMOS transistor connected to the drain of the fifth PMOS transistor, and the gate of the first NMOS transistor connected to the gate of the fifth PMOS transistor and to the collector of the fifth npn bipolar transistor.
 48. The circuit of claim 47, wherein the CMOS buffer further comprises:(iii) a sixth PMOS transistor comprising a source, a drain and a gate, the source of the sixth PMOS transistor capable of receiving the supply voltage; and (iv) a second NMOS transistor comprising a source, a drain and a gate, the source of the second NMOS transistor capable of being grounded, the drain of the second NMOS transistor connected to the drain of the sixth PMOS transistor, and the gate of the second NMOS transistor connected to the gate of the sixth PMOS transistor and to the drains of the first NMOS transistor and the fifth PMOS transistor.
 49. The circuit of claim 48, wherein the power-on reset output is connected to the drains of the second NMOS transistor and the sixth PMOS transistor.
 50. The circuit of claim 35, wherein the voltage of the power-on reset pulse is equal to the supply voltage up to the predetermined threshold voltage.
 51. A power-on reset circuit, comprising:(a) a self-biased current generator capable of receiving a supply voltage and generating a current which is proportional to an absolute temperature in the self-biased current generator in response to receiving the supply voltage, the self-biased current generator comprising:(i) a first p-channel metal oxide semiconductor (PMOS) transistor comprising a source, a drain and a gate, the source of the first PMOS transistor capable of receiving the supply voltage; (ii) a second PMOS transistor comprising a source, a drain and a gate, the source of the second PMOS transistor capable of receiving the supply voltage, and the gate of the second PMOS transistor connected to the drain of the second PMOS transistor and to the gate of the first PMOS transistor; (iii) a first npn bipolar transistor comprising an emitter, a collector and a base, the emitter of the first npn bipolar transistor capable of being grounded, and the collector of the first npn bipolar transistor connected to the base of the first npn bipolar transistor and to the drain of the first PMOS transistor; (iv) a second npn bipolar transistor comprising an emitter, a collector and a base, the collector of the second npn bipolar transistor connected to the drain of the second PMOS transistor, and the base of the second npn bipolar transistor connected to the base of the first npn bipolar transistor; and (v) a first resistor coupled to the emitter of the second npn bipolar transistor, the second npn bipolar transistor capable of supplying the current which is proportional to the absolute temperature to the first resistor in response to receiving the supply voltage by the sources of the first and second PMOS transistors; (b) a base-emitter voltage detector, coupled to the self-biased current generator, comprising:(i) a third PMOS transistor comprising a source, a drain and a gate, the source of the third PMOS transistor capable of receiving the supply voltage, and the gate of the third PMOS transistor connected to the gate of the second PMOS transistor; (ii) a third npn bipolar transistor comprising an emitter, a collector and a base, the collector of the third npn bipolar transistor connected to the drain of the third PMOS transistor and to the base of the third npn bipolar transistor; and (iii) a fourth npn bipolar transistor comprising an emitter, a collector and a base, the emitter of the fourth npn bipolar transistor capable of being grounded; (iv) a second resistor connected between the emitter of the third npn bipolar transistor and the collector of the fourth npn bipolar transistor; and (v) a capacitor coupled to the emitter of the third npn bipolar transistor; (c) a bipolar complementary metal oxide semiconductor (BiCMOS) inverter coupled to the base-emitter voltage detector; (d) a complementary metal oxide semiconductor (CMOS) buffer coupled to the BiCMOS inverter and capable of generating a power-on reset pulse having a voltage proportional to the supply voltage up to a predetermined threshold voltage; and (e) a power-on reset output coupled to the CMOS buffer and capable of outputting the power-on reset pulse from the CMOS buffer.
 52. The circuit of claim 51, wherein the emitters of the first, third and fourth npn bipolar transistors have an equal emitter area.
 53. The circuit of claim 52, wherein the emitter of the second npn bipolar transistor has an emitter area larger than that of the emitter of the first npn bipolar transistor.
 54. The circuit of claim 51, wherein the first, second and third PMOS transistors have an equal shape factor.
 55. The circuit of claim 51, wherein the first resistor comprises a first terminal connected to the emitter of the second npn bipolar transistor and a second terminal capable of being grounded.
 56. The circuit of claim 51, wherein the capacitor comprises a first terminal connected to the emitter of the third npn bipolar transistor and a second terminal capable of being grounded.
 57. The circuit of claim 51, wherein the BiCMOS inverter comprises:(i) a fourth PMOS transistor comprising a source, a drain and a gate, the source of the fourth PMOS transistor capable of receiving the supply voltage, and the gate of the fourth PMOS transistor connected to the gate of the third PMOS transistor; and (ii) a fifth npn bipolar transistor comprising an emitter, a collector and a base, the collector of the fifth npn bipolar transistor connected to the drain of the fourth PMOS transistor, the emitter of the fifth npn bipolar transistor capable of being grounded, and the base of the fifth npn bipolar transistor connected to the collector of the fourth npn bipolar transistor.
 58. The circuit of claim 57, wherein the emitters of the first and fifth npn bipolar transistors have an equal emitter area.
 59. The circuit of claim 57, wherein the fourth PMOS transistor has a shape factor of about 1/4 of that of the first PMOS transistor.
 60. The circuit of claim 51, wherein the CMOS buffer comprises:(i) a fifth PMOS transistor comprising a source, a drain and a gate, the source of the fifth PMOS transistor capable of receiving the supply voltage; and (ii) a first n-channel metal oxide semiconductor (NMOS) transistor comprising a source, a drain and a gate, the source of the first NMOS transistor capable of being grounded, the drain of the first NMOS transistor connected to the drain of the fifth PMOS transistor, and the gate of the first NMOS transistor connected to the gate of the fifth PMOS transistor and to the collector of the fifth npn bipolar transistor.
 61. The circuit of claim 60, wherein the CMOS buffer further comprises:(iii) a sixth PMOS transistor comprising a source, a drain and a gate, the source of the sixth PMOS transistor capable of receiving the supply voltage; and (iv) a second NMOS transistor comprising a source, a drain and a gate, the source of the second NMOS transistor capable of being grounded, the drain of the second NMOS transistor connected to the drain of the sixth PMOS transistor, and the gate of the second NMOS transistor connected to the gate of the sixth PMOS transistor and to the drains of the first NMOS transistor and the fifth PMOS transistor.
 62. The circuit of claim 61, wherein the power-on reset output is connected to the drains of the second NMOS transistor and the sixth PMOS transistor.
 63. The circuit of claim 51, wherein the voltage of the power-on reset pulse is equal to the supply voltage up to the predetermined threshold voltage.
 64. A power-on reset circuit, comprising:(a) a self-biased current generator capable of receiving a supply voltage and generating a current which is proportional to an absolute temperature in the self-biased current generator in response to receiving the supply voltage, the self-biased current generator comprising:(i) a first p-channel metal oxide semiconductor (PMOS) transistor comprising a source, a drain and a gate, the source of the first PMOS transistor capable of receiving the supply voltage; (ii) a second PMOS transistor comprising a source, a drain and a gate, the source of the second PMOS transistor capable of receiving the supply voltage, and the gate of the second PMOS transistor connected to the drain of the second PMOS transistor and to the gate of the first PMOS transistor; (iii) a first npn bipolar transistor comprising an emitter, a collector and a base, the emitter of the first npn bipolar transistor capable of being grounded, and the collector of the first npn bipolar transistor connected to the base of the first npn bipolar transistor and to the drain of the first PMOS transistor; (iv) a second npn bipolar transistor comprising an emitter, a collector and a base, the collector of the second npn bipolar transistor connected to the drain of the second PMOS transistor, and the base of the second npn bipolar transistor connected to the base of the first npn bipolar transistor; and (v) a first resistor coupled to the emitter of the second npn bipolar transistor, the second npn bipolar transistor capable of supplying the current which is proportional to the absolute temperature to the first resistor in response to receiving the supply voltage by the sources of the first and second PMOS transistors; (b) a base-emitter voltage detector, coupled to the self-biased current generator, comprising:(i) a third PMOS transistor comprising a source, a drain and a gate, the source of the third PMOS transistor capable of receiving the supply voltage, and the gate of the third PMOS transistor connected to the gate of the second PMOS transistor; (ii) a third npn bipolar transistor comprising an emitter, a collector and a base, the collector of the third npn bipolar transistor connected to the drain of the third PMOS transistor and to the base of the third npn bipolar transistor; and (iii) a fourth npn bipolar transistor comprising an emitter, a collector and a base, the emitter of the fourth npn bipolar transistor capable of being grounded, and the base of the fourth npn bipolar transistor connected to the base of the second npn bipolar transistor; (iv) a second resistor connected between the emitter of the third npn bipolar transistor and the collector of the fourth npn bipolar transistor; and (v) a capacitor coupled to the emitter of the third npn bipolar transistor; (c) a bipolar complementary metal oxide semiconductor (BiCMOS) inverter coupled to the base-emitter voltage detector, the BiCMOS inverter comprising:(i) a fourth PMOS transistor comprising a source, a drain and a gate, the source of the fourth PMOS transistor capable of receiving the supply voltage, and the gate of the fourth PMOS transistor connected to the gate of the third PMOS transistor; and (ii) a fifth npn bipolar transistor comprising an emitter, a collector and a base, the collector of the fifth npn bipolar transistor connected to the drain of the fourth PMOS transistor, the emitter of the fifth npn bipolar transistor capable of being grounded, and the base of the fifth npn bipolar transistor connected to the collector of the fourth npn bipolar transistor; (d) a complementary metal oxide semiconductor (CMOS) buffer coupled to the BiCMOS inverter and capable of generating a power-on reset pulse having a voltage proportional to the supply voltage up to a predetermined threshold voltage; and (e) a power-on reset output coupled to the CMOS buffer and capable of outputting the power-on reset pulse from the CMOS buffer.
 65. The circuit of claim 64, wherein the emitters of the first, third, fourth and fifth npn bipolar transistors have an equal emitter area.
 66. The circuit of claim 65, wherein the emitter of the second npn bipolar transistor has an emitter area larger than that of the emitter of the first npn bipolar transistor.
 67. The circuit of claim 64, wherein the first, second and third PMOS transistors have an equal shape factor.
 68. The circuit of claim 67, wherein the fourth PMOS transistor has a shape factor of about 1/4 of that of the first PMOS transistor.
 69. The circuit of claim 64, wherein the first resistor comprises a first terminal connected to the emitter of the second npn bipolar transistor and a second terminal capable of being grounded.
 70. The circuit of claim 64, wherein the capacitor comprises a first terminal connected to the emitter of the third npn bipolar transistor and a second terminal capable of being grounded.
 71. The circuit of claim 64, wherein the CMOS buffer comprises:(i) a fifth PMOS transistor comprising a source, a drain and a gate, the source of the fifth PMOS transistor capable of receiving the supply voltage; and (ii) a first n-channel metal oxide semiconductor (NMOS) transistor comprising a source, a drain and a gate, the source of the first NMOS transistor capable of being grounded, the drain of the first NMOS transistor connected to the drain of the fifth PMOS transistor, and the gate of the first NMOS transistor connected to the gate of the fifth PMOS transistor and to the collector of the fifth npn bipolar transistor.
 72. The circuit of claim 71, wherein the CMOS buffer further comprises:(iii) a sixth PMOS transistor comprising a source, a drain and a gate, the source of the sixth PMOS transistor capable of receiving the supply voltage; and (iv) a second NMOS transistor comprising a source, a drain and a gate, the source of the second NMOS transistor capable of being grounded, the drain of the second NMOS transistor connected to the drain of the sixth PMOS transistor, and the gate of the second NMOS transistor connected to the gate of the sixth PMOS transistor and to the drains of the first NMOS transistor and the fifth PMOS transistor.
 73. The circuit of claim 72, wherein the power-on reset output is connected to the drains of the second NMOS transistor and the sixth PMOS transistor.
 74. The circuit of claim 64, wherein the voltage of the power-on reset pulse is equal to the supply voltage up to the predetermined threshold voltage. 